1. Field of the Invention
The invention relates to a flat cable, and a connection structure between a flat cable and a printed wiring board.
2. Description of the Related Art
Conventionally, a wire harness is used as a wiring component for electrically connecting plural printed wiring boards which are mounted inside, e.g., an on-vehicle inverter unit or an engine control unit, and a connection structure using a connector component is employed for connection between the wire harness and the printed wiring board. In recent years, use of an alternative wiring component in place of wire harness, an application of a connection method not using a connector component and simplification of connection process are required as a measure of realizing both downsizing/thinning of and cost reduction of on-vehicle devices.
In order to respond to such downsizing and cost reduction of on-vehicle devices, an inter-board connection structure has been proposed in which a flat cable called FFC (Flexible Flat Cable) including plural conductors arranged in parallel, e.g., a conductor potion formed of a Cu alloy (oxygen-free copper, tough pitch copper), which are integrated by adhesively covering a covering insulation film using an adhesive material from both sides of the conductor portion in a thickness direction is employed as a wiring component used in an on-vehicle device. In the FFC, a exposed conductor portion which is exposed from the insulation film is formed at both longitudinal ends of the conductor, and is connected to an electrode section of a printed wiring board. And also, MFJ (Multi Frame Joiner) and FPC (Flexible Print Circuit), etc., are employed for a flat cable used as a wiring component in an on-vehicle device.
For connection between the exposed conductor portion of the flat cable and the electrode section provided on the printed wiring board, a structure of direct connection using a joining material such as solder material or conductive adhesive material not through a connector may be employed. A direct connection using a solder material, etc., allows not only downsizing in accordance with a decrease in a connecting area and reduction of the number of connecting parts but also reduction or simplification of attachment processes by simultaneously performing the direct connection with solder connection of electronic component attached to the printed wiring board other than the wiring component.
On the other hand, high durable reliability for long time use has been required for on-vehicle devices. Ensuring of reliability against long-term vibration load or thermal load is also vital for a wiring component attached to an on-vehicle device or a connecting portion thereof. In a wiring component for connecting plural printed wiring boards, mechanical load repeatedly acts on a connecting portion of the wiring component due to, resonant vibration of the wiring component itself, etc., caused by vibration load acting on the on-vehicle device. There is a high possibility that a fatigue fracture occurs at the connecting portion of the wiring component due to the mechanical load, hence, it is especially important to ensure reliability against vibration load in a wiring component for on-vehicle devices.
Ensuring of long-term reliability is vital for on-vehicle devices, and a flat cable itself and a connecting portion thereof are also required to ensure reliability against vibration load or thermal load. Particularly, reliability against mechanical load such as vibration or impact is important for on-vehicle devices which are mounted inside an engine compartment. In order to improve reliability, it is necessary to optimize the entire structure of the on-vehicle device and also to study a structure or means which reduces mechanical load applied to the connecting portion of the flat cable and improves resistance against mechanical load.
The inter-board wiring component to connect a exposed conductor portion of a flat cable to an electrode section of a printed wiring board using a solder material has a structure in which load is likely to be applied to the vicinity of the connecting portion of the exposed conductor portion. Large stress is concentrated especially on a exposed conductor portion at a covering material end portion or an upper end portion of a solder connection fillet at the tip of the exposed conductor portion.
When mechanical load, especially high amplitude mechanical load in a thickness direction of the flat cable (a direction to separate a connection interface between the electrode section of the printed wiring board and the exposed conductor portion of the flat cable) acts on the connecting portion between the electrode section of the printed wiring board and the exposed conductor portion of the flat cable, fracture or separation of the connecting portion or breaking of the exposed conductor portion of the flat cable may occur.
As a method of reducing mechanical load applied to the connecting portion between the exposed conductor portion of the flat cable and the electrode section of the printed wiring board, a method is suggested in which a flat wiring material restricting clip is provided to restrict a flat wiring material such as FFC or FPC to a circuit board and the flat wiring material is pressed down on the circuit board at a portion closer to the edge of the circuit board than to the conductor end portion of the flat wiring material by the flat wiring material restricting clip in a state that the conductor of the flat wiring material is connected to the circuit board (see, e.g., JP-A-2001-143784).
According to the means of pressing down the flat wiring material on the circuit board by the flat wiring material restricting clip in a state that the conductor of the flat wiring material is connected to the circuit board as disclosed in JP-A-2001-143784, when an external mechanical force in a separating direction is applied to the connecting portion of the flat wiring material, it is possible to prevent the external mechanical force from acting on the connecting portion by restriction of the flat wiring material restricting clip. As a result, it is possible to prevent damage to the connecting portion between the circuit board and the flat wiring material.
Meanwhile, as a means of reinforcing a connecting portion between a conductor of a flat cable and a circuit of a printed wiring board, a method is suggested in which adhesion between the flat cable and the printed wiring board is enhanced to reinforce the connecting portion therebetween (see, e.g., JP-A-H8-203577). According to this conventional method, a right-angle bent portion is formed on a conductor at an end portion of the FFC and an end portion of the conductor of the FFC is inserted into a hole formed on a corresponding circuit of the printed wiring board (FPC, etc.). Then, the conductor of the FFC is fixed to the back surface of the FPC by pressure bonding or soldering and is reinforced from both sides of the conductor by plastic reinforcing plates or by holding with an adhesive tape.
According to the means of reinforcing the connecting portion between the conductor of the flat cable and the circuit of the printed wiring board as disclosed in JP-A-H8-203577, the reinforcing plates sandwich or the adhesive tape is wound multiple times around the flat cable as well as the printed wiring board from both upper and lower sides to fix the conductor of the flat cable to the circuit of the printed wiring board at the connecting portion, and it is thereby possible to reduce external mechanical force which acts on the connecting portion.
In addition, as a means of connecting and fixing a flat cable or a cable of a flexible wiring board, etc., to a printed wiring board, a method in which a fixing plate (a plate formed of metal) for applying pressure to a cable placed on a printed wiring board is provided at an upper portion of the cable and is fixed to the printed wiring board by a screw, or a method in which a cable is fixed to a printed wiring board by inserting a terminal having a claw formed at a tip thereof into a hole provided on the printed wiring board is suggested (see, e.g., JP-A-2002-216873).
According to the means of fixing a flat cable or a cable of a flexible wiring board to a printed wiring board as disclosed in JP-A-2002-216873, the fixing board which covers the connecting portion between a conductor of the cable and the printed wiring board can be fixed to the printed wiring board by a terminal having a claw formed at a tip thereof, and it is thereby possible to reduce external mechanical force which acts on the connecting portion.